There are a variety of known applications where it is necessary or desirable to communicate data over power lines. For example, electric power meters for measuring power consumption in a building can advantageously be read remotely over the same power lines to which they are coupled. (See, e.g., U.S. Pat. No. 5,844,949). In another application, appliances within a household, such as a light switch or dimmer, can be controlled by signalling placed on the household power lines.
Conventional power lines, however, typically present a high-distortion, high-noise environment in which reliable and fast data communication is often not possible. In addition to additive white Gaussian noise (AWGN), power lines exhibit noise that is synchronous with the line frequency (e.g., lamp dimmer noise), periodic noise (e.g., motor-generated noise), random noise, radio frequency (RF) noise and interference from other communications devices such as intercoms and security systems. In addition to noise, data signals may also experience substantial attenuation such as when traveling through a transformer from one phase to another. Moreover, notch filters or other frequency-selective attenuation may be coupled to the power lines such as in power supplies for personal computers or power strips.
An additional problem involves voltage transformers, which are inherently inductively coupled and thereby introduce non-linear phase shifts in a signal passing through the transformer. This can lead to substantial signal distortion.
Furthermore, much of the noise on typical household power lines is located at harmonics of the AC line frequency (i.e., 50 or 60 Hz) that extend over large frequency ranges. As such, data communication over virtually any band of frequencies wider than the line frequency will be susceptible to such harmonic noise.
Known power line communications protocols (e.g., X-10, CEBus) often do not provide reliable operation to all outlets in a typical home. Moreover, such known techniques often require costly bridging devices to allow devices coupled to circuits on different phases to communicate with each other.
Complex communications systems are required to perform reliably in the high-noise, phase-distorted power line environment. For example, some of these systems monitor different frequency channels, select an appropriate channel, and then indicate the appropriate frequency to other communicating units. Some systems employ a plurality of signals which are not harmonically related. This requires complicated filtering and signal extraction. (See, e.g., U.S. Pat. No. 5,185,591.) Another scheme, Geometric Harmonic Modulation (GHM) allocates signaling energy into lobes, or tones, at different frequencies being evenly spaced at geometrically increasing multiples of a base frequency. The GHM signaling waveforms are spread spectrum signals in that the signal bandwidth, the bandwidth from the lowest frequency tone to the highest, vastly exceeds the information bandwidth conveyed by the GHM transmission.
Such solutions, however, can be costly to implement and may not ensure fool proof operation, thus rendering them inappropriate for applications where both cost and reliability are motivating considerations such as in household appliance control and communications.
There thus exists a need for a communication system which can reliably yet cost-effectively provide data communications over power lines even under the most adverse conditions.